A number of oncofoetal or tumour-associated antigens (TAAs) have been identified and characterised in human and animal tumours. In general, TAAs are antigens expressed during foetal development which are downregulated in adult cells, and are thus normally absent or present only at very low levels in adults. Tumour cells have been observed to resume expression of TAAs, and the application of TAAs for tumour diagnosis, targeting and immunotherapy has therefore been suggested.
In particular, the recent cloning of tumour antigens recognised by T cells has caused considerable interest in the development of antigen specific cancer vaccines. However, many tumour associated antigens are non-mutated, poorly immunogenic tissue differentiation antigens. Their weak immunogenicity may be due to self tolerance. Thus they are rarely indicated as antigenic peptides suitable for raising an immune response.
Notwithstanding this, some tumour associated antigens are found to be regularly associated with tumours in a large number of individuals. Such antigens are especially attractive candidates for use in vaccines. They include the melanoma differentiation antigens (MDA), melanoma antigens which are recognised by T lymphocytes as well as several proteins in the MAGE family. However, as indicated by results from clinical trials obtained to date, inducing therapeutic T cells to these antigens has proved extremely difficult. One reason for the apparent hyporesponsiveness of the human immune system to many tumour antigens may be that they are normal, non-mutated self antigens, expressed on normal tissues as well as on tumour cells. The immune system is not able to differentiate the tumour antigen on a tumour cell from ordinary, self proteins.
A major barrier to the application of tumour immunotherapy approaches using non-mutated self cellular antigens is thus apparently the breaking of tolerance to such an antigen. For example, a murine zona pellucida antigen expressed by a murine poxvirus recombinant was able to induce infertility in mice. These data indicate that though the breaking of tolerance using recombinant pox viruses expressing self antigens is possible, there is still a requirement to optimise their efficacy such that the active treatment of established tumours becomes possible.
The TAA 5T4 (see WO 89/07947) has been extensively characterised. It is a 72 kDa glycoprotein expressed widely in carcinomas, but having a highly restricted expression pattern in normal adult tissues (see Table 1). It appears to be strongly correlated to metastasis in colorectal and gastric cancer. The full nucleic acid sequence of human 5T4 is known (Myers et al., 1994 J Biol Chem 169: 9319-24).
TABLE 1Distribution of Human 5T4Tumour Type5T4 Frequency (%)Breast84Ovarian71Gastric74Colorectal85
(Starzynska et al., Eur J Gastroenterol Hepatol June 1998;10(6):479-84; Starzynska et al., Br. J Cancer May 1994;69(5):899-902; Starzynska et al., Br J Cancer November 1992;66(5):867-9)
Although 5T4 has been proposed as a marker, with possible mechanistic involvement, for tumour progression and metastasis potential (Carsberg et al., (1996) Int J Cancer Sep. 27, 1996;68(1):84-92), 5T4 has not been proposed for use as an immunotherapeutic agent. The breaking of immune tolerance to 5T4, which is itself expressed in a restricted manner in adult tissues, has not been demonstrated. Thus, it could not be predicted whether 5T4 could prove to be an effective antigen for immunotherapy against cancer.